widget: fix Geometry::expand using other.height twice

The bounding-box height was computed from other.height + other.height
instead of other.height + other.y, so the combined damage rectangle sent
to the compositor was wrong whenever a widget's drawn area changed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/widgets/widget.rs

@@ -40,7 +40,7 @@ impl Geometry {
             x,
             y,
             width: max(self.width + self.x, other.width + other.x) - x,
-            height: max(self.height + self.y, other.height + other.height) - y,
+            height: max(self.height + self.y, other.height + other.y) - y,
         }
     }
 
@@ -64,3 +64,30 @@ pub trait Widget {
     fn draw(&mut self, fonts: &mut FontMap, view: &mut BufferView) -> Geometry;
     fn event(&mut self, _event: &Event) {}
 }
+
+#[cfg(test)]
+mod tests {
+    use super::Geometry;
+
+    #[test]
+    fn expand_is_bounding_box() {
+        let a = Geometry {
+            x: 10,
+            y: 20,
+            width: 5,
+            height: 5,
+        };
+        let b = Geometry {
+            x: 0,
+            y: 100,
+            width: 2,
+            height: 2,
+        };
+        let e = a.expand(b);
+        assert_eq!((e.x, e.y), (0, 20));
+        assert_eq!(e.x + e.width, 15);
+        assert_eq!(e.y + e.height, 102);
+        let e2 = b.expand(a);
+        assert_eq!((e2.x, e2.y, e2.width, e2.height), (e.x, e.y, e.width, e.height));
+    }
+}